Abstract
Classical Fc receptors (FcRs) mediate the binding to and recognition of the Fc portion of antibodies and play an important role during immune responses in mammals. Although proteins similar to soluble FcRs have been identified in fish, little is known about the role of such proteins in fish immunity. Here, we cloned a cDNA sequence encoding a soluble Fc receptor for an immunoglobulin G (FcγR) homolog from ayu (Plecoglossus altivelis) (PaFcγRl). The predicted protein was composed of two immunoglobulin C2-like domains but lacked a transmembrane segment and a cytoplasmic tail. The PaFcγRl transcripts were distributed at low levels in all tested tissues, but significantly increased after Vibrio anguillarum infection. The PaFcγRl protein was expressed in the head kidney, trunk kidney, and neutrophils. Recombinant PaFcγRl (rPaFcγRl) was secreted when transfected into mammalian cells and the native protein was also detected in serum upon infection. rPaFcγRl was also demonstrated to bind to ayu IgM, as assessed by cell transfection. Suppressive activity of the recombinant mature protein of PaFcγRl (rPaFcγRlm) on in vitro anti-sheep red blood cell (SRBC) responses was detected by a modified hemolytic plaque forming cell assay. In conclusion, our study revealed that PaFcγRl is closely involved in the negative regulation of IgM production in the ayu spleen.
Highlights
Antibodies are key components of the immune system, linking both innate and adaptive immunity (Bournazos & Ravetch, 2015)
It was predicted to comprise of a signal peptide and two Ig domains: Ig domain 1 (D1) consisting of 80 aa
Myeloid leukocytes in humans and mice are equipped with a variety of receptors that enable their interaction with monomeric or aggregated immunoglobulins, antigen-antibody immune complexes, and opsonized antibody-coated particles or cells (Pierre & Friederike, 2015)
Summary
Antibodies are key components of the immune system, linking both innate and adaptive immunity (Bournazos & Ravetch, 2015). Antibodies contain a constant region, termed the Fc domain, which engages diverse cellular receptors, thereby triggering antibodymediated effector functions in innate and adaptive immunity (Dilillo & Ravetch, 2015). As members of the immunoglobulin (Ig) superfamily, Fc receptors (FcRs) are broadly expressed on the surface of various myeloid leukocytes and mediate binding and recognition of the Fc portion of antibodies (Davis, 2007). Since their identification three decades ago, our understanding of the biological consequences of FcRs has continued to evolve. FcRs mediate various functions including phagocytosis, antibody-dependent cell-mediated cytotoxicity, lymphocyte proliferation, mast cell degranulation, release/ secretion of cytokines and chemokines, antigen presentation, Received: 18 February 2019; Accepted: 26 June 2019; Online: 23 July
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